The present invention relates to directional control of a hovercraft (also commonly known as an “air-cushion vehicle,” or “ACV”), more particularly to systems and devices involving discharge of high-velocity air for maneuvering a hovercraft.
Many air-cushion vehicles are equipped with bow thrusters. The United States Navy's Landing Craft Air Cushion (LCAC) is an amphibious landing vehicle that implements air-cushion technology and that is primarily used to transport weapons, equipment, cargo, and personnel from ship to shore. FIG. 1 depicts a representative LCAC 50, including two bow thrusters 100 (one port, one starboard). Bow thruster (“BT”) systems are important components of the control system of an LCAC, and are especially useful at lower speeds for negotiating crowded waters, docking, or approaching shore.
High-velocity air generated by the port and starboard lift fans not only lifts the LCAC, but also is ducted to each of the two bow thrusters. High-velocity air leaves the nozzle of each bow thruster, thus representing thrust that augments directional control, provided by both rudder and propeller, of the craft. Typically, each bow thruster 100 includes a rotatable 90-degree nozzle 130, and is used as a thrust-generation and directional-control device. Centrifugal fans provide pressurized air to the nozzle, which is rotatable to eject air in 360-degree directions to generate thrust for directional controls.
With reference to FIG. 2 through FIG. 6, the performance of the nozzle interior flow depends on the nozzle inflow conditions and the nozzle flow-path design. Since the nozzle is to be rotatable for the directional control, the nozzle entrance (inlet) 131 usually has a circular shape. In furtherance of design efficiency (e.g., aerodynamic efficiency), the nozzle exit (outlet) 132 usually has an elliptical shape. An inlet flow straightener portion 141 of horizontal vane 135 extends into inlet 131 and serves to improve the entering flow. A blending shape section 133 is developed between the nozzle inlet 131 and the nozzle outlet 132 to convert the circular cross-section to the elliptical cross-section.
An example of a conventional 90-degree bow thruster nozzle 130 is designated herein “BT-16” and is depicted in FIG. 2 through FIG. 5. The BT-16 nozzle 130 is shown connected, at the nozzle inlet 131, to a slanted non-nozzle duct 140. In order to maintain the originally designed cross-sectional shapes of the nozzle wall 139 along the nozzle flow path, vertical and horizontal vanes (such as vertical vane 134 and horizontal vane 135, shown in FIG. 3 and FIG. 5) are traditionally installed inside the nozzle wall 139, in the nozzle flow path, to be used as structural stiffeners. These vanes usually cause flow re-circulation and separation from the attached surfaces, such as illustrated in FIG. 2 through FIG. 4. The bow thruster 100 designated herein “BT-15” (not shown) has the same nozzle 130 as bow thruster BT-16, but with a chamber-type plenum connected to the nozzle inlet 131.
Another example of a 90-degree bow thruster nozzle 130 that implements both horizontal and vertical vanes is designated herein “BT-17” and is depicted in FIG. 6. Vis-à-vis the BT-16, the BT-17 effects some local modification of the vertical vane 134. As compared with the BT-16 nozzle (FIG. 5), the BT-17 nozzle 130 (FIG. 6) has a shorter vertical vane 134 along the vertical vane 134 leading edge. That is, the BT-17's vertical vane 134 vertically extends between the horizontal vane 135 and the top area of the nozzle wall 139, over approximately fifty percent of the minor diameter of the nozzle outlet 132. By comparison, the vertical vane 134 of the BT-16 extends over the entire minor diameter of the nozzle outlet 132. In addition to being abbreviated in the vertical direction vis-à-vis the BT-16's vertical vane 134, the leading edge of the BT-17's vertical vane 134 is characterized by a different vertical angle from that of the BT-16's vertical vane 134.
In either the BT-16 design or the BT-17 design, the horizontal vane 135 can be considered to divide the nozzle 130 region in the vicinity of nozzle outlet 132 into upper and lower nozzle portions, viz., an upper nozzle component 136 and a lower nozzle component 137. The vertical vane 134 of the BT-16 vertically spans both the upper nozzle component 136 and the lower nozzle component 137 of the nozzle 130. In contrast, the vertical vane 134 of the BT-17 vertically spans only the upper nozzle component 136 of the nozzle 130. For the lower nozzle component 137 of the BT-17 nozzle 130, the vertical vane 134 is replaced by two stiffener struts 138 at the leading and trailing edges, respectively, of the horizontal vane 135, thereby reducing flow separation, and hence reducing the aerodynamic losses by about six percent, vis-à-vis the BT-16 nozzle 130. Although the two struts 138 are intended to lend structural support to the nozzle 130, the construction of the BT-17 in terms of its abbreviated vertical vane 134 has not been verified for its structural integrity.